Plastic encapsulated semiconductor devices are susceptible to moisture ingress due to the permeable nature of plastic molding compounds and printed circuit (PC) board substrate materials. The amount of moisture that a plastic resin encapsulated semiconductor device absorbs from its environment is dependent on several factors: the length of exposure time to the environment, the moisture level in the environment, the diffusivity of the plastic or how quickly moisture can be absorbed into the material, the solubility coefficient of the plastic or its saturation capacity, and the thickness of the plastic body on the device. Devices containing moisture levels exceeding some critical amount run the risk of cracking, delaminating, or "popcorning" during the rapid heating of the solder reflow operation associated with board mounting of devices because the moisture inside the device will vaporize causing a rapid increase in vapor pressure internal to the device.
Semiconductor devices which are subject to popcorning are normally baked in an oven at approximately 125.degree. C., a typical temperature, for a predetermined length of time to drive moisture out of the devices before they are shipped to the customer. Those devices that are deemed to be moisture sensitive are packaged in "dry-packs" after baking to ensure that they are protected from moisture thereafter and will arrive dry at the customer site. Otherwise, devices that have absorbed a certain level of moisture run the risk of popcorning during the solder reflow operation. Mechanical failure of the semiconductor devices often times lead to subsequent electrical failure of these same devices due to thermal and mechanical stresses induced on the devices during their operation.
Current dry-packing practices involve baking semiconductor devices until dry, placing them into a dry-pack bag with desiccant packets and a humidity indicator card, vacuum sealing the bag immediately thereafter, and shipping the devices to the customer in these dry-packs. Typically, devices remain in a dry-pack bag until they are needed for board mounting. Once devices are removed from the dry-pack bag, they must be board mounted within a limited time to ensure that they have not reabsorbed enough moisture from the environment to popcorn during solder reflow. This time limitation is disadvantageous because it restricts the time window for the user to board mount all of the devices shipped in one dry-pack bag. If the time limit is exceeded, then the devices that have not yet been board mounted have to be rebaked to ensure that they will not popcorn during the solder reflow operation.
One type of plastic semiconductor device that is very susceptible to the popcorn phenomena is an overmolded pad array carrier (also known as ball grid array or BGA) semiconductor device. These devices are more prone to popcorning than other plastic encapsulated devices such as plastic dual-in-line packages (PDIPs), quad flat packs (QFPs), plastic leaded chip carriers (PLCCs), etc. The reason for this susceptibility is that these overmolded devices are thin, and are composed of organic materials that allow rapid moisture diffusion into the package.
The general construction of an overmolded pad array carrier has a semiconductor die mounted on an upper surface of a substrate and a plurality of solder balls attached to a lower surface of the substrate. The semiconductor die is overmolded with an encapsulating resin material which forms a package body on the upper surface of the substrate. The substrate is a PC board material, such as BT resin clad with copper. A vent hole can be placed in the bottom of the package underneath the die to act as a pressure relief path for moisture vapor. This vent hole is desirable because it has been shown to reduce the incidence of package popcorning during solder reflow. However, implementing a manufacturable process for assembling an overmolded leadless pad array carrier with a vent hole has not been practical or easy because of inherent complications with the design.
One problem with the vent hole concept is that the hole is located approximately in the center of the die mounting area of the substrate. The problem arises in the step of mounting the die on the substrate. The adhesive typically used to mount the die is a silver filled epoxy paste, which will flow into the vent hole unless somehow controlled. It is not desirable to have this adhesive paste fill the vent hole because it defeats the function of the vent hole which is to provide a pressure relief path for moisture. Furthermore, the adhesive paste can also contaminate the substrate, especially the solder pads on the lower surface of the substrate which could cause solderability problems when attaching the plurality of solder bumps to the substrate. Therefore, to prevent adhesive paste from flowing into the vent hole, the vent hole must be sealed during the die mounting process. If the vent hole is sealed, then the adhesive paste cannot flow into the vent hole during the die mounting process because air is trapped inside the vent hole due to the seal.
However, to have the vent hole function as intended, the seal must be removed, punctured or broken after the die mounting process step. Only if the seal is broken will there be a channeled pressure relief path for moisture to prevent the overmolded pad array carrier from popcorning during board mounting. Currently, the only methods for breaking this seal on the vent hole are either mechanical piercing or laser drilling, both of which have major disadvantages.
A disadvantage with the mechanical piercing method is that the vent hole is very small; therefore precise locating of the vent hole is required to prevent damage to the device during the piercing operation. This method is difficult to implement in a production environment because the machine parameters have to be tightly controlled at all times to ensure accurate piercing location and depth of piercing.
Although laser drilling can accurately open the vent hole seal consistently, the cost of implementing laser drilling is prohibitive. Laser drilling is a highly expensive, capital intensive method just to open a vent hole seal. Moreover, the laser drilling equipment requires a lot of floor space on the production floor which can be difficult to accommodate.
Another disadvantage to either proposed method for opening the vent hole seal is that both methods constitute an additional operation in the assembly of the overmolded pad array carrier with a vent hole. An added assembly operation translates into longer production cycle time and increased cost in the device. There is also a possibility for yield loss due to the extra operation. Furthermore, a sampled inspection of the devices must be implemented after this operation which is another non-value added operation.